In cooperation with Fort Peck Tribes Office of Environmental Protection

Delineation of Brine Contamination in and near the East Poplar Oil Field, Fort Peck Indian Reservation, Northeastern Montana, 2004–09

By Joanna N. Thamke and Bruce D. Smith

Abstract

The extent of brine contamination in the shallow aquifers in and near the East Poplar oil field is as much as 17.9 square miles and appears to be present throughout the entire saturated zone in contaminated areas. The brine contamination affects 15–37 billion gallons of groundwater. Brine contamination in the shallow aquifers east of the Poplar River generally moves to the southwest toward the river and then southward in the Poplar River valley. The likely source of brine contamination in the shallow aquifers is brine that is produced with crude oil in the East Poplar oil field study area. Brine contamination has not only affected the water quality from privately owned wells in and near the East Poplar oil field, but also the city of Poplar’s public water-supply wells.

Three water-quality types characterize water in the shallow aquifers; a fourth water-quality type in the study area characterizes the brine. Type 1 is uncontaminated water that is suitable for most domestic purposes and typically contains sodium bicarbonate and sodium/magnesium sulfate as the dominant ions. Type 2 is moderately contaminated water that is suitable for some domestic purposes, but not used for drinking water, and typically contains sodium and chloride as the dominant ions. Type 3 is considerably contaminated water that is unsuitable for any domestic purpose and always contains sodium and chloride as the dominant ions. Type 3 quality of water in the shallow aquifers is similar to Type 4, which is the brine that is produced with crude oil.

Electromagnetic apparent conductivity data were collected in the 106 square-mile area and used to determine extent of brine contamination. These data were collected and interpreted in conjunction with water-quality data collected through 2009 to delineate brine plumes in the shallow aquifers. Monitoring wells subsequently were drilled in some areas without existing water wells to confirm most of the delineated brine plumes; however, several possible plumes do not contain either existing water wells or monitoring wells. Analysis of groundwater samples from wells confirms the presence of 12.1 square miles of contamination, as much as 1.7 square miles of which is considerably contaminated (Type 3). Electromagnetic apparent conductivity data in areas with no wells delineate an additional 5.8 square miles of possible contamination, 2.1 square miles of which might be considerably contaminated (Type 3). Storage-tank facilities, oil wells, brine-injection wells, pipelines, and pits are likely sources of brine in the study area. It is not possible to identify discrete oil-related features as likely sources of brine plumes because several features commonly are co-located. During the latter half of the twentieth century, many brine plumes migrated beyond the immediate source area and likely mix together in modern and ancestral Poplar River valley subareas.

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Contents

Appendix 1. Ground electromagnetic measurements in and near the East Poplar oil field, 2005

Appendix 2. Lithologic logs and completion details for U.S. Geological Survey and Fort Peck Tribes Office of Environmental Protection monitoring wells drilled in and near the East Poplar oil field, 2006 and 2009

Appendix 3. Data from aquifer test conducted in Quaternary alluvium in the East Poplar oil field, March 27, 2007

Appendix 4. Data from aquifer tests conducted in Quaternary Wiota Gravel in the East Poplar oil field, March 29–30 and June 26–27, 2007

Appendix 5. Physical and chemical characteristics of water samples collected from privately owned wells, monitoring wells, oil wells, and brine-injection wells in and near the East Poplar oil field, 1952–2009

Appendix 6. Physical and chemical characteristics of rinsate water after field cleaning of U.S. Geological Survey sampling equipment before and during water-sample collection from wells in and near the East Poplar oil field, September 2003 and March 2007

Appendix 7. Concentration of selected fuel ethers, petroleum compounds, and radionuclides in water samples collected from selected monitoring wells and in rinsate water after cleaning of sampling equipment in and near the East Poplar oil field, March 2007

Appendix 8. Estimating rates of water drainage through the unsaturated zone at the East Poplar oil field on the basis of concentration profiles of chloride and nitrate, 2006 by Richard W. Healy, Branch of Regional Research, Central Region, U.S. Geological Survey

Appendix 9. Sample depth, water content by weight, and chloride and nitrate concentrations in water and sediment extracts in and near the East Poplar oil field, August 2006

Appendix 10. Average chloride and nitrate in soil water and drainage rates for 11 sites in and near the East Poplar oil field, August 2006